bootfs: need to use fakeroot for tar'ing

Unpacking the boot.tar.gz file using sudo caused error:
"Cannot change ownership to uid 1000, gid 1000: Operation not permitted"
Now using fakeroot to not record my uid to the files.

Change-Id: I1a953e5e363a0d9fbb914cfa8837d888421d8e14
Reviewed-by: Samuli Piippo <samuli.piippo@digia.com>